Recently, in general, Group III nitride semiconductors made of compounds of N with Al, Ga, In, and so forth are widely used in light-emitting elements, elements for electronic devices, and the like. The characteristics of such devices are greatly dependent on the crystallinity of Group III nitride semiconductors, making techniques for growing highly crystalline Group III nitride semiconductors necessary.
Group III nitride semiconductors have been formed conventionally by epitaxial growth on sapphire substrates. However, sapphire substrates have poor heat dissipation due to low thermal conductivity, which is not suitable for producing high-power output devices.
Therefore, in recent years, a technique for using a silicon substrate (Si substrate) as a substrate for crystal growth of a Group III nitride semiconductor has been proposed. Si substrates have better heat dissipation than the above sapphire substrates and are therefore suitable for producing high-power output devices. Furthermore, since large substrates are inexpensive, they are advantageous in reducing production cost. However, like sapphire substrates, Si substrates have different lattice constants than Group III nitride semiconductors. Therefore, growing a Group III nitride semiconductor directly on such a Si substrate is not expected to provide a highly crystalline Group III nitride semiconductor.
Furthermore, a Group III nitride semiconductor has a high thermal expansion coefficient in comparison with silicon. Accordingly, when this Group III nitride semiconductor is grown directly on a Si substrate, a great tensile strain occurs in the Group III nitride semiconductor in the course of cooling from the high temperature of a crystal growth process to room temperature. This leads to problems of the Si substrate warping, and to the generation of high-density cracks in the Group III nitride semiconductor.
Therefore, JP2007-67077A (PTL 1) discloses a technique for manufacturing, on a Si substrate, a highly crystalline Group III nitride semiconductor in which generation of cracks is prevented by providing an AlN-based superlattice buffer layer between the silicon substrate and the Group III nitride semiconductor. The AlN-based superlattice buffer layer has a plurality of alternately stacked first layers, made of AlxGa1-xN (the Al composition ratio x being such that 0.5≤x≤1) and second layers, made of AlyGa1-yN (the Al composition ratio y being such that 0.01≤y≤0.2).